The invention relates to a circuit for an FM-receiver having a frequency-locked loop, whose loop gain is negative at least for the low frequencies in the demodulated signal. The circuit includes an FM-demodulator, a tunable oscillator circuit whose frequency is controlled by the FM-demodulator output signals and a mixing stage mixing the oscillator signal with an input signal. The output of this mixing stage is coupled to the input of the FM-demodulator.
Such a circuit is disclosed in DE-OS No. 3032701. The total FM-demodulator output-signal not just its DC-component, is employed to control the oscillator frequency. As in the case of tuning to a transmitter, the oscillator frequency changes continuously in accordance with the LF-output signal and more specifically in such a way that the frequency swing of the frequency-modulated input signal is reduced. It is therefore possible to choose an intermediate frequency the magnitude of which is equal or even smaller than that of the frequency swing. For FM-receivers whose input signal has a frequency swing of 75 kHz an intermediate frequency of for example 70 kHz may be chosen. This has the advantage that filtering can be effected in the intermediate-frequency range by means of a simple RC low-pass filter which does not require adjustment.
This prior art FM-receiver has the drawback that the amplitude of the output signal and the transmission bandwidth are highly dependent on the tuning frequency of the oscillator when the oscillator frequency is controlled by the FM-demodulator in known manner with a variable capacity diode.
The invention has for its object a circuit of the above-mentioned type when the amplitude of the output signal and the transmission bandwidth are substantially independent of the tuning frequency of the oscillator.